Contactless power transmission system for transmitting power from power transmitter apparatus to power receiver apparatus, and supplying load device with desired voltage

ABSTRACT

A detection circuit detects at least one of a value of a current flowing through a power transmitting coil, and a value of a current or voltage generated by an auxiliary coil. A control circuit determines a transmitting frequency based on the value detected by the detection circuit, the transmitting frequency at least locally minimizing load dependence. The control circuit determines a voltage for the transmitting power at which an output voltage of a power receiver apparatus is equal to a predetermined target voltage when generating the transmitting power having the transmitting frequency determined, and controls the power supply circuit to generate the transmitting power having the transmitting frequency and voltage determined.

CROSS REFERENCE TO RELATED APPLICATIONS

This is the U.S. national stage of application No. PCT/JP2019/046552,filed on Nov. 28, 2019. Priority under 35 U.S.C. § 119(a) and 35 U.S.C.§ 365(b) is claimed from Japanese Application No. 2019-003687, filedJan. 11, 2019, the disclosure of which is also incorporated herein byreference.

TECHNICAL FIELD

The present disclosure relates to a control apparatus for a powertransmitter apparatus that transmits power to a power receiver apparatusin a contactless manner. The present disclosure further relates to apower transmitter apparatus provided with such a control apparatus, andrelates to a contactless power transmission system including such apower transmitter apparatus.

BACKGROUND ART

Contactless power transmission systems are known, which transmit powerfrom a power transmitter apparatus to a power receiver apparatus in acontactless manner. When the power transmitter apparatus transmits powerto the power receiver apparatus in a contactless manner, the powerreceiver apparatus is not always placed at a fixed position relative tothe power transmitter apparatus. Therefore, a distance between a powertransmitting coil of the power transmitter apparatus and a powerreceiving coil of the power receiver apparatus may vary, and a couplingcoefficient between the power transmitting coil and the power receivingcoil may vary accordingly. When the coupling coefficient between thepower transmitting coil and the power receiving coil varies, a voltageand/or current supplied from the power receiver apparatus to a loaddevice varies accordingly.

In order for the power receiver apparatus to supply the load device witha desired voltage thereof, it may be possible to, for example, feed backan output voltage value and/or output current value of the powerreceiver apparatus to the power transmitter apparatus, and regulate avoltage applied to the power transmitting coil. In addition, in orderfor the power receiver apparatus to supply the load device with adesired voltage thereof, it may be possible to, for example, provide thepower receiver apparatus with a DC/DC converter.

For example, Patent Document 1 discloses a non-contact power supplysystem that supplies a desired output voltage and output current to thebattery by feeding back values of an output voltage value and an outputcurrent for a battery of a vehicle, from the vehicle to a power supplydevice, and regulating an output voltage and a drive frequency of thepower supply device.

CITATION LIST Patent Documents

-   PATENT DOCUMENT 1: Japanese patent No. JP 6201388 B1

SUMMARY OF INVENTION Technical Problem

When the output voltage value and/or output current value of the powerreceiver apparatus is fed back to the power transmitter apparatus, somedelay occurs, and it is difficult to follow rapid variations in a loadvalue of the power receiver apparatus (a magnitude of a current flowingthrough the load device). In addition, when the power receiver apparatusis provided with a DC/DC converter, the size, weight, and cost of thepower receiver apparatus increase. Therefore, it is required to controlthe power transmitter apparatus to supply the load device with a desiredvoltage thereof, without depending on the feedback from the powerreceiver apparatus to the power transmitter apparatus, and withoutproviding the power receiver apparatus with extra circuits (DC/DCconverter or the like).

Therefore, an object of the present disclosure is to provide a controlapparatus for a power transmitter apparatus, the control apparatus beingcapable of controlling only the power transmitter apparatus based onlyon information that can be acquired by the power transmitter apparatus,to supply a load device with a desired voltage thereof.

Another object of the present disclosure is to provide a powertransmitter apparatus having such a control apparatus, and to furtherprovide a contactless power transmission system including such a powertransmitter apparatus.

Solution to Problem

The control apparatus for the power transmitter apparatus, the powertransmitter apparatus, and the contactless power transmission systemaccording to aspects of the present disclosure are configured asdescribed below to solve the above-described problems.

According to a control apparatus for a power transmitter apparatus of anaspect of the present disclosure, the control apparatus for a powertransmitter apparatus transmits power to a power receiver apparatusprovided with a power receiving coil, in a contactless manner. The powertransmitter apparatus is provided with: a power transmitting coil, anauxiliary coil electromagnetically coupled to the power transmittingcoil, and a power supply circuit that generates transmitting powerhaving a variable voltage and a variable frequency and supplies thetransmitting power to the power transmitting coil. The control apparatusis provided with a detection circuit and a control circuit. Thedetection circuit detects at least one of a value of a current flowingthrough the power transmitting coil, and a value of a current or voltagegenerated by the auxiliary coil. The control circuit determines atransmitting frequency based on the value detected by the detectioncircuit, the transmitting frequency at least locally minimizingdependence of an output voltage of the power receiver apparatus on aload value of the power receiver apparatus, determines a voltage for thetransmitting power at which the output voltage of the power receiverapparatus is equal to a predetermined target voltage when generating thetransmitting power having the transmitting frequency determined, andcontrols the power supply circuit to generate the transmitting powerhaving the transmitting frequency and voltage determined.

Since the control apparatus for the power transmitter apparatus of theaspect of the present disclosure is configured as described above, it ispossible to supply the load device with a desired voltage thereof,substantially without depending on the load value of the power receiverapparatus, by controlling only the power transmitter apparatus basedonly on the information that can be acquired by the power transmitterapparatus.

According to the control apparatus for the power transmitter apparatusof the aspect of the present disclosure, the power receiver apparatus isprovided with a first load device having a variable load value, a secondload device having a predetermined load value, and a switch circuit thatselectively supplies the output voltage of the power receiver apparatusto one of the first load device and the second load device. The controlapparatus is further provided with a communication devicecommunicatively connected to the power receiver apparatus. Whentransmitting power in a normal manner, the control circuit transmits asignal to the power receiver apparatus using the communication device,the signal controlling the switch circuit to supply the output voltageof the power receiver apparatus to the first load device. Whendetermining the transmitting frequency, the control circuit transmits asignal to the power receiver apparatus using the communication device,the signal controlling the switch circuit to supply the output voltageof the power receiver apparatus to the second load device, anddetermines the transmitting frequency based on the value detected by thedetection circuit.

Since the control apparatus for the power transmitter apparatus of theaspect of the present disclosure is configured as described above, it ispossible to correctly determine whether or not a foreign object existsbetween the power transmitting coil and the power receiving coil, evenwhen the load device has a variable load value.

According to the control apparatus for the power transmitter apparatusof the aspect of the present disclosure, the control apparatus isfurther provided with a coupling coefficient estimator that estimates acoupling coefficient between the power transmitting coil and the powerreceiving coil based on the value detected by the detection circuit, anddetermines the transmitting frequency based on the coupling coefficient.

Since the control apparatus for the power transmitter apparatus of theaspect of the present disclosure is configured as described above, it ispossible to supply the load device with a desired voltage thereof,substantially without depending on the load value of the power receiverapparatus, by controlling only the power transmitter apparatus basedonly on the information that can be acquired by the power transmitterapparatus.

According to the control apparatus for the power transmitter apparatusof the aspect of the present disclosure, the detection circuit isprovided with a first detector that detects the value of the current orvoltage generated by the auxiliary coil, and a second detector thatdetects the current flowing through the power transmitting coil. Thecoupling coefficient estimator estimates a first coupling coefficientbetween the power transmitting coil and the power receiving coil basedon the value of the current or voltage generated by the auxiliary coil,and estimates a second coupling coefficient between the powertransmitting coil and the power receiving coil based on the value of thecurrent flowing through the power transmitting coil. When a differencebetween the first and second coupling coefficients is equal to or lessthan a predetermined threshold, the control circuit controls the powersupply circuit to generate the transmitting power having the frequencyand voltage determined.

Since the control apparatus for the power transmitter apparatus of theaspect of the present disclosure is configured as described above, it ispossible to correctly and surely determine whether or not a foreignobject exists between the power transmitting coil and the powerreceiving coil, and continue power transmission when no foreign objectexists.

According to the control apparatus for the power transmitter apparatusof the aspect of the present disclosure, when the difference between thefirst and second coupling coefficients is greater than the predeterminedthreshold, the control circuit controls the power supply circuit to stoppower transmission to the power receiver apparatus.

Since the control apparatus for the power transmitter apparatus of theaspect of the present disclosure is configured as described above, it ispossible to correctly and surely determine whether or not a foreignobject exists between the power transmitting coil and the powerreceiving coil,

According to a power transmitter apparatus of an aspect of the presentdisclosure, the power transmitter apparatus is provided with: a powertransmitting coil; an auxiliary coil electromagnetically coupled to thepower transmitting coil; a power supply circuit that generatestransmitting power having a variable voltage and a variable frequencyand supplies the transmitting power to the power transmitting coil; andthe control apparatus for the power transmitter apparatus.

Since the power transmitter apparatus of the aspect of the presentdisclosure is configured as described above, it is possible to supplythe load device with a desired voltage thereof, substantially withoutdepending on the load value of the power receiver apparatus, bycontrolling only the power transmitter apparatus based only on theinformation that can be acquired by the power transmitter apparatus.

According to the power transmitter apparatus of the aspect of thepresent disclosure, the power transmitter apparatus is further providedwith a capacitor connected to the power transmitting coil so as to forman LC resonant circuit.

Since the power transmitter apparatus of the aspect of the presentdisclosure is configured as described above, it is possible to adjust again in output voltage of the power receiver apparatus, and improveefficiency of power transmission.

According to the power transmitter apparatus of the aspect of thepresent disclosure, the power transmitter apparatus is further providedwith a magnetic core around which the power transmitting coil and theauxiliary coil are wound. The auxiliary coil is disposed so as tosurround the power transmitting coil.

Since the power transmitter apparatus of the aspect of the presentdisclosure is configured as described above, it is possible to increasemagnetic flux density of the power transmitting coil, and reduce leakagemagnetic flux.

According to a contactless power transmission system of an aspect of thepresent disclosure, the contactless power transmission system includes:the power transmitter apparatus; and a power receiver apparatus providedwith a power receiving coil.

Since the contactless power transmission system of the aspect of thepresent disclosure is configured as described above, it is possible tosupply the load device with a desired voltage thereof, substantiallywithout depending on the load value of the power receiver apparatus, bycontrolling only the power transmitter apparatus based only on theinformation that can be acquired by the power transmitter apparatus.

Advantageous Effects of Invention

According to the present disclosure, by controlling only the powertransmitter apparatus based only on the information that can be acquiredby the power transmitter apparatus, it is possible to supply the loaddevice with a desired voltage thereof, substantially without dependingon the load value of the power receiver apparatus.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram showing an exemplary configuration of acontactless power transmission system according to a first embodiment.

FIG. 2 is a perspective view of an exemplary configuration of magneticcores F1 and F2 shown in FIG. 1.

FIG. 3 is a diagram showing an application example of the contactlesspower transmission system shown in FIG. 1.

FIG. 4 is a graph showing an example of a change in magnitude of acurrent I3 generated by an auxiliary coil L3 and detected by a detector13 shown in FIG. 1.

FIG. 5 is a graph showing an example of a change in magnitude of acurrent I1 flowing through a power transmitting coil L1 and detected bya detector 14 shown in FIG. 1.

FIG. 6 is a table showing examples of a coupling coefficient k12 betweenthe power transmitting coil L1 and a power receiving coil L2, thecoupling coefficient k12 being calculated for the current I1 flowingthrough the power transmitting coil L1 and the current I3 generated bythe auxiliary coil L3 shown in FIG. 1.

FIG. 7 is a flowchart showing a first power transmission processexecuted by a control circuit 16 shown in FIG. 1.

FIG. 8 is a flowchart showing a second power transmission processexecuted by a control circuit 16 of a contactless power transmissionsystem according to a second embodiment.

FIG. 9 is an exemplary graph illustrating that an output voltage of apower receiver apparatus 20 shown in FIG. 1 varies depending on thecoupling coefficient k12 between the power transmitting coil L1 and thepower receiving coil L2, and depending on a voltage V0 of a powertransmitter apparatus 10.

FIG. 10 is an exemplary graph illustrating that the output voltage ofthe power receiver apparatus 20 shown in FIG. 1 varies depending on thecoupling coefficient k12 between the power transmitting coil L1 and thepower receiving coil L2, and depending on a load value of a load device22.

FIG. 11 is a flowchart showing a third power transmission processexecuted by a control circuit 16 of a contactless power transmissionsystem according to a first modified embodiment of the secondembodiment.

FIG. 12 is a flowchart showing a fourth power transmission processexecuted by a control circuit 16 of a contactless power transmissionsystem according to a second modified embodiment of the secondembodiment.

DESCRIPTION OF EMBODIMENTS

Hereinafter, embodiments according to one aspect of the presentdisclosure (hereinafter, also referred to as “the present embodiments”)will be described with reference to the drawings. In the drawings, thesame reference sign indicates similar components.

First Embodiment

A contactless power transmission system according to a first embodimentwill be described with reference to FIG. 1 to FIG. 7.

Application Example of First Embodiment

It is required that a contactless power transmission system, whichtransmits power from a power transmitter apparatus to a power receiverapparatus in a contactless manner, detect a foreign object locatedbetween the power transmitter apparatus and the power receiverapparatus. In order to detect such a foreign object located between thepower transmitter apparatus and the power receiver apparatus, it isconsidered that, for example, the power receiver apparatus calculates aratio of magnitude of a current actually received by the power receiverapparatus, to a current value notified from the power transmitterapparatus, and requests the power transmitter apparatus to stop powertransmission when the ratio is less than a predetermined value. However,in this case, some delay occurs for feedback from the power receiverapparatus to the power transmitter apparatus. Therefore, it is difficultto detect a foreign object before generating heat, and stop powertransmission. Alternatively, it is considered to detect a foreign objectusing a camera, or detect a temperature increase due to a foreign objectusing a temperature sensor. However, in this case, the size, weight, andcost of the power transmitter apparatus and/or the power receiverapparatus increase. In addition, when trying to detect a foreign objectbased on only a change in one circuit parameter of the power transmitterapparatus, it is difficult to distinguish whether the change occurs dueto an influence of the foreign object, or due to a change in couplingcoefficient caused by a change in distance between a power transmittingcoil and a power receiving coil. Accordingly, the foreign object cannotbe detected, and if continuing power transmission, the foreign objectmay be heated. Therefore, it is required to surely detect a foreignobject through a simple configuration without a camera, a temperaturesensor, or the like, and without depending on feedback from the powerreceiver apparatus to the power transmitter apparatus.

According to the first embodiment, a contactless power transmissionsystem will be described, which is capable of surely detecting a foreignobject through a simple configuration, without depending on feedbackfrom a power receiver apparatus to a power transmitter apparatus.

FIG. 1 is a block diagram showing an exemplary configuration of acontactless power transmission system according to the first embodiment.The contactless power transmission system shown in FIG. 1 includes apower transmitter apparatus 10 and a power receiver apparatus 20, andthe power transmitter apparatus 10 transmits power to the power receiverapparatus 20 in a contactless manner.

The power transmitter apparatus 10 is provided with at least an AC/DCconverter 11, an inverter 12, detectors 13 and 14, a couplingcoefficient estimator 15, a control circuit 16, a power transmittingcoil L1, and an auxiliary coil L3.

The AC/DC converter 11 converts an AC voltage inputted from an AC powersource 1, into a DC voltage V0 having a certain magnitude. The inverter12 operates at a certain switching frequency fsw, to convert the DCvoltage V0 inputted from the AC/DC converter 11, into an AC voltage V1.The voltage V1 is applied across the power transmitting coil L1. Theamplitude of the voltage V1 is equal to the magnitude of the voltage V0.

In the present specification, the AC/DC converter 11 and the inverter 12are collectively referred to as a “power supply circuit”. In otherwords, the power supply circuit generates transmitting power having apredetermined voltage and frequency, and supplies the transmitting powerto the power transmitting coil L1.

When the power transmitter apparatus 10 transmits power to the powerreceiver apparatus 20, the power transmitting coil L1 iselectromagnetically coupled to a power receiving coil L2 (to bedescribed later) of the power receiver apparatus 20. In addition, theauxiliary coil L3 is electromagnetically coupled to the powertransmitting coil L1.

The detector 13 detects a value of a current I3 or a voltage V3generated by the auxiliary coil L3. The detector 14 detects a value of acurrent I1 flowing through the power transmitting coil L1. The valuesdetected by the detectors 13 and 14 are notified to the control circuit16.

In the present specification, the detector 13 is also referred to as a“first detector”, and the detector 14 is also referred to as a “seconddetector”. In addition, in the present specification, the detectors 13and 14 are collectively referred to as a “detection circuit”.

The coupling coefficient estimator 15 estimates a first couplingcoefficient k12 a between the power transmitting coil L1 and the powerreceiving coil L2, based on the value of the current I3 or voltage V3generated by the auxiliary coil L3. The coupling coefficient estimator15 estimates a second coupling coefficient k12 b between the powertransmitting coil L1 and the power receiving coil L2, based on the valueof the current I1 flowing through the power transmitting coil L1.

The control circuit 16 controls the AC/DC converter 11 and the inverter12 to start and stop generating the transmitting power.

The power receiver apparatus 20 is provided with at least the powerreceiving coil L2. A load device 22 is provided inside or outside thepower receiver apparatus 20. The load device 22 includes, for example, abattery, a motor, an electric circuit, and/or an electronic circuit. Thepower received from the power transmitter apparatus 10 via the powerreceiving coil L2 is supplied to the load device 22.

In the present specification, a voltage V4 applied to the load device 22is also referred to as an “output voltage” of the power receiverapparatus 20.

When a difference between the first coupling coefficient k12 a and thesecond coupling coefficient k12 b is equal to or less than apredetermined threshold, the control circuit 16 of the power transmitterapparatus 10 controls the AC/DC converter 11 and the inverter 12 totransmit power to the power receiver apparatus 20. When the differencebetween the first coupling coefficient k12 a and the second couplingcoefficient k12 b is greater than the threshold, the control circuit 16controls the AC/DC converter 11 and the inverter 12 to stop powertransmission to the power receiver apparatus 20. Here, the threshold isdetermined such that the first coupling coefficient k12 a and the secondcoupling coefficient k12 b are considered to be substantially equal toeach other.

According to the first embodiment, the detectors 13 and 14, the couplingcoefficient estimator 15, and the control circuit 16 are collectivelyreferred to as a “control apparatus” of the power transmitter apparatus10.

A coupling coefficient k12 between the power transmitting coil L1 andthe power receiving coil L2 varies depending on a distance between thepower transmitting coil L1 and the power receiving coil L2. The couplingcoefficient k12 increases as the distance decreases, and the couplingcoefficient k12 decreases as the distance increases. In addition, thecurrent I1 flowing through the power transmitting coil L1 varies withcertain characteristics, depending on the coupling coefficient k12between the power transmitting coil L1 and the power receiving coil L2.The current I3 (and/or voltage V3) generated by the auxiliary coil L3varies with characteristics different from those of the current I1,depending on the coupling coefficient k12 between the power transmittingcoil L1 and the power receiving coil L2. The coupling coefficientestimator 15 stores, in advance, a table or formulas indicating arelationship between the current I1 and the coupling coefficient k12,and a relationship between the current I3 (or voltage V3) and thecoupling coefficient k12. The coupling coefficient estimator 15 canestimate the coupling coefficients k12 a and k12 b based on the valuesof the currents I1 and I3 (alternatively, the current I1 and voltageV3), with reference to such a table or formulas. When no foreign objectexists between the power transmitting coil L1 and the power receivingcoil L2, it is expected that the estimated coupling coefficients k12 aare k12 b are equal to each other. On the other hand, when a foreignobject exists between the power transmitting coil L1 and the powerreceiving coil L2, the currents I1 and I3 are differently affected bythe foreign object, and as a result, the estimated coupling coefficientsk12 a and k12 b are different from each other.

When a power transmitter apparatus transmits power to a power receiverapparatus in a contactless manner, the power receiver apparatus is notalways placed at a fixed position relative to the power transmitterapparatus. For example, consider a case where the power receiverapparatus is an electrically-driven vehicle with a battery, and thepower transmitter apparatus is a charging stand for the vehicle. In thiscase, due to a misalignment of the vehicle from a position in front ofthe charging stand, or due to a change in distance between the chargingstand and the vehicle, deviations of, for example, several millimetersto several tens of millimeters may occur each time the vehicle stops atthe charging stand. Accordingly, a distance between the powertransmitting coil of the power transmitter apparatus and the powerreceiving coil of the power receiver apparatus may vary, and therefore,a coupling coefficient between the power transmitting coil and the powerreceiving coil may vary. When trying to detect a foreign object based ononly a change in one circuit parameter of the power transmitterapparatus, it is difficult to distinguish whether the change occurs dueto an influence of the foreign object, or due to a change in couplingcoefficient caused by a change in distance between the powertransmitting coil and the power receiving coil.

On the other hand, according to the contactless power transmissionsystem of the first embodiment, the first coupling coefficient k12 aestimated based on the value of the current I3 or voltage V3 generatedby the auxiliary coil L3 is compared with the second couplingcoefficient k12 b estimated based on the value of the current I1 flowingthrough the power transmitting coil L1, as described above. In otherwords, according to the contactless power transmission system of thefirst embodiment, two circuit parameters of the power transmitterapparatus 10 are used in order to detect a foreign object. Thus, it ispossible to surely determine whether or not a foreign object existsbetween the power transmitting coil L1 and the power receiving coil L2,regardless of a change in coupling coefficient between the powertransmitting coil L1 and the power receiving coil L2. As describedabove, according to the contactless power transmission system of thefirst embodiment, it is possible to surely detect a foreign objectthrough a simple configuration without a camera, a temperature sensor,or the like, and without depending on feedback from the power receiverapparatus 20 to the power transmitter apparatus 10. By not depending onfeedback from the power receiver apparatus 20 to the power transmitterapparatus 10, it is possible to detect a foreign object beforegenerating heat, and facilitate to stop power transmission. In addition,by not including a camera, a temperature sensor, or the like, it ispossible to reduce the size, weight, and cost of the power transmitterapparatus 10 and the power receiver apparatus 20, or at least, make thesize, weight, and cost less likely to increase.

According to the first embodiment, the power receiver apparatus 20 maybe an electronic device with a battery (for example, a laptop computer,a tablet computer, a mobile phone, or the like), and the powertransmitter apparatus 10 may be a charger for the power receiverapparatus 20. In addition, according to the first embodiment, the powerreceiver apparatus 20 may be an electrically-driven vehicle with abattery (for example, an electric vehicle or an automated guidedvehicle), and the power transmitter apparatus 10 may a charging standfor the power receiver apparatus 20. In addition, according to the firstembodiment, the power receiver apparatus 20 may be a pallet thatrequires a power source for performing some work on a load duringtransportation, and the power transmitter apparatus 10 may be a conveyorcapable of supplying power to such pallets. In addition, the firstembodiment is applicable to a contactless power transmission system inwhich the distance between the power transmitting coil L1 and the powerreceiving coil L2 is fixed. In this case, for example, the powertransmitter apparatus 10 and the power receiver apparatus 20 may beprovided instead of a slip ring, at a joint of a robot arm or the like,in order to supply power to a drive mechanism located at a tip of therobot arm or the like.

Configuration Example of First Embodiment

As shown in FIG. 1, the power transmitter apparatus 10 receives powerfrom the AC power source 1. The AC power source 1 is, for example, acommercial power source.

In the example shown in FIG. 1, the power transmitter apparatus 10 isprovided with the AC/DC converter 11, the inverter 12, the detectors 13and 14, the coupling coefficient estimator 15, the control circuit 16, acommunication device 17, a capacitor C1, a magnetic core F1, the powertransmitting coil L1, the auxiliary coil L3, and a current detectionresistor R1.

The AC/DC converter 11 converts the AC voltage inputted from the ACpower source 1, into the DC voltage V0, as described above. The AC/DCconverter 11 may convert the AC voltage inputted from the AC powersource 1, into a DC voltage V0 having a magnitude variable under thecontrol of the control circuit 16. The AC/DC converter 11 may beprovided with a power factor correction circuit. The inverter 12converts the DC voltage V0 inputted from the AC/DC converter 11, intothe AC voltage V1, as described above. The inverter 12 generates, forexample, a rectangular AC voltage V1 at the switching frequency fsw. Theinverter 12 may operate at a switching frequency fsw variable under thecontrol of the control circuit 16.

The power transmitter apparatus 10 may be provided with the capacitorC1. In this case, the capacitor C1 is connected to the powertransmitting coil L1 so as to form an LC resonant circuit. By providingthe capacitor C1, it is possible to adjust a gain in output voltage ofthe power receiver apparatus 20, and improve efficiency of powertransmission.

The power transmitter apparatus 10 may be provided with the magneticcore F1. In this case, the power transmitting coil L1 and the auxiliarycoil L3 are wound around the magnetic core F1. By winding the powertransmitting coil L1 around the magnetic core F1, it is possible toincrease magnetic flux density of the power transmitting coil L1, andreduce leakage magnetic flux.

The detector 13 detects the value of the current I3 or voltage V3generated by the auxiliary coil L3, as described above. The detector 14uses the current detection resistor R1 to detect the value of thecurrent I1 flowing through the power transmitting coil L1.

The coupling coefficient estimator 15 estimates the first couplingcoefficient k12 a between the power transmitting coil L1 and the powerreceiving coil L2, based on the value of the current I3 or voltage V3generated by the auxiliary coil L3, as described above. In addition, thecoupling coefficient estimator 15 estimates the second couplingcoefficient k12 b between the power transmitting coil L1 and the powerreceiving coil L2, based on the value of the current I1 flowing throughthe power transmitting coil L1, as described above.

The control circuit 16 controls the AC/DC converter 11 and the inverter12 to start and stop generating the transmitting power, as describedabove. When determining that no foreign object exists between the powertransmitting coil L1 and the power receiving coil L2, the controlcircuit 16 controls the AC/DC converter 11 and the inverter 12 totransmit power to the power receiver apparatus 20. When determining thata foreign object exists between the power transmitting coil L1 and thepower receiving coil L2, the control circuit 16 stops the AC/DCconverter 11 and the inverter 12. The control circuit 16 may regulatethe magnitude of the DC voltage V0 outputted from the AC/DC converter11, and the switching frequency fsw of the inverter 12. The controlcircuit 16 includes a central processing unit (CPU), a random accessmemory (RAM), a read only memory (ROM), and the like, and executes afirst power transmission process to be described later.

The power transmitter apparatus 10 may be provided with thecommunication device 17. In this case, the communication device 17 iscommunicatively connected to a communication device 24 (to be describedlater) of the power receiver apparatus 20 by radio (for example,infrared ray) or by wire. The control circuit 16 may receive a signalfrom the power receiver apparatus 20 via the communication device 17,the signal indicating that the power receiver apparatus 20 has requestedpower transmission. In addition, the control circuit 16 may receive asignal from the power receiver apparatus 20 via the communication device17, the signal indicating a value of a voltage and/or current to besupplied to the load device 22, and the like. When the power receiverapparatus 20 has a normal mode and a test mode (to be described later),the control circuit 16 transmits a signal to the power receiverapparatus 20 via the communication device 17, the signal requestingtransition to the test mode, or transition to the normal mode.

In the example shown in FIG. 1, the power receiver apparatus 20 isprovided with a rectifier circuit 21, the load device 22, a controlcircuit 23, a communication device 24, a capacitor C2, a magnetic coreF2, the power receiving coil L2, a load element R2, and a switch circuitSW.

When the power transmitter apparatus 10 transmits power to the powerreceiver apparatus 20, the power receiving coil L2 iselectromagnetically coupled to the power transmitting coil L1 togenerate a current I2 and voltage V2 in the power receiving coil L2.

The power receiver apparatus 20 may be provided with the capacitor C2.In this case, the capacitor C2 is connected to the power receiving coilL2 so as to form an LC resonant circuit. By providing the capacitor C2,it is possible to adjust a gain in output voltage of the power receiverapparatus 20, and improve efficiency of power transmission.

The power receiver apparatus 20 may be provided with the magnetic coreF2. In this case, the power receiving coil L2 is wound around themagnetic core F2. By winding the power receiving coil L2 around themagnetic core F2, it is possible to increase magnetic flux density ofthe power receiving coil L2, and reduce leakage magnetic flux.

The rectifier circuit 21 converts the AC voltage V2 inputted from thepower receiving coil L2, into the DC voltage V4. The rectifier circuit21 may be provided with a smoothing circuit and/or a power factorcorrection circuit.

The power receiver apparatus 20 may be provided with the control circuit23, the communication device 24, the load element R2, and the switchcircuit SW. In this case, the voltage V4 outputted from the rectifiercircuit 21 is selectively supplied to the load device 22 or the loadelement R2, via the switch circuit SW operating under the control of thecontrol circuit 23. For example, when the load device 22 is a battery,the load device 22 has a variable load value that varies depending on astate of charge of the battery. On the other hand, the load element R2has a predetermined load value. Here, the load value indicates, forexample, the magnitude of the current flowing through the load device 22or the load element R2. The load element R2 and the switch circuit SWare configured in a manner simpler than that of, for example, a DC/DCconverter, and so as to be less likely to affect the efficiency of powertransmission to the load device 22. The load element R2 may have a loadvalue smaller than the load value of the load device 22. The powerreceiver apparatus 20 has the normal mode where the voltage V4 outputtedfrom the rectifier circuit 21 is supplied to the load device 22, and thetest mode where the voltage V4 outputted from the rectifier circuit 21is supplied to the load element R2. The communication device 24 iscommunicably connected to the communication device 17 of the powertransmitter apparatus 10 by radio (for example, infrared ray) or bywire, as described above. The control circuit 23 receives a signal fromthe power transmitter apparatus 10 via the communication device 24, thesignal requesting transition to the test mode, or transition to thenormal mode.

When transmitting power in a normal manner, the control circuit 16 ofthe power transmitter apparatus 10 transmits a signal to the powerreceiver apparatus 20 using the communication device 17, the signalrequesting transition to the normal mode (that is, a signal controllingthe switch circuit SW to supply the output voltage of the power receiverapparatus 20 to the load device 22).

When the load device 22 has a variable load value, the current I1flowing through the power transmitting coil L1 varies with certaincharacteristics depending on the load value of the load device 22, evenwhen the coupling coefficient k12 between the power transmitting coil L1and the power receiving coil L2 is constant. Similarly, the current I3(and/or voltage V3) generated by the auxiliary coil L3 also varies withcharacteristics different from those of the current I1, depending on theload value of the load device 22. Accordingly, when the load value ofthe load device 22 varies, it is not possible to accurately estimate thecoupling coefficient k12 between the power transmitting coil L1 and thepower receiving coil L2, and therefore, it is not possible to correctlydetermine whether or not a foreign object exists between the powertransmitting coil L1 and the power receiving coil L2. Therefore, thecontrol circuit 16 of the power transmitter apparatus 10 transmits asignal to the power receiver apparatus 20, the signal requestingtransition to the test mode (that is, a signal controlling the switchcircuit SW to supply the output voltage of the power receiver apparatus20 to the load element R2), and estimates the coupling coefficients k12a and k12 b based on the values detected by the detectors 13 and 14 whenthe power receiver apparatus 20 is in the test mode. Thus, even when theload device 22 has a variable load value, it is possible to correctlydetermine whether or not a foreign object exists between the powertransmitting coil L1 and the power receiving coil L2.

In the present specification, the load device 22 is also referred to asa “first load device”, and the load element R2 is also referred to as a“second load device”.

The control circuit 23 of the power receiver apparatus 20 may transmit asignal to the power transmitter apparatus 10 via the communicationdevice 24, the signal indicating that the power receiver apparatus 20has requested power transmission. In addition, the control circuit 23may transmits a signal to the power transmitter apparatus 10 via thecommunication device 24, the signal indicating a value of a voltageand/or current to be supplied to the load device 22.

The voltage generated in the power receiver apparatus 20 (voltage V4outputted from the rectifier circuit 21, or the like) varies dependingon the coupling coefficient k12 between the power transmitting coil L1and the power receiving coil L2. The voltage increases as the couplingcoefficient k12 increases, and the voltage decreases as the couplingcoefficient k12 decreases. The circuit parameters of the powertransmitter apparatus 10 and the power receiver apparatus 20 aredetermined so as to prevent overvoltage in the power receiver apparatus20, even when the power transmitter apparatus 10 and the power receiverapparatus 20 operate at a frequency at which the coupling coefficientk12 between the power transmitting coil L1 and the power receiving coilL2 is maximized, and at which the voltage V4 is maximized or locallymaximized.

FIG. 2 is a perspective view of an exemplary configuration of themagnetic cores F1 and F2 shown in FIG. 1. As described above, the powertransmitting coil L1 and the auxiliary coil L3 may be wound around themagnetic core F1, and the power receiving coil L2 may be wound aroundthe magnetic core F2. A part of the magnetic flux generated from thepower transmitting coil L1 intersects the auxiliary coil L3 to generatethe current I3 and voltage V3 in the auxiliary coil L3. In addition, asshown in FIG. 2, the auxiliary coil L3 may be disposed so as to surroundthe power transmitting coil L1. By disposing the auxiliary coil L3 insuch a manner, it is possible to reduce leakage magnetic flux of thepower transmitting coil L1.

The power transmitting coil L1 and the power receiving coil L2 areelectromagnetically coupled to each other with the coupling coefficientk12, the power transmitting coil L1 and the auxiliary coil L3 areelectromagnetically coupled to each other with a coupling coefficientk13, and the power receiving coil L2 and the auxiliary coil L3 areelectromagnetically coupled to each other with a coupling coefficientk23. The power transmitting coil L1, the power receiving coil L2, andthe auxiliary coil L3 are configured such that the coupling coefficientsk13 and k23 are much smaller than the coupling coefficient k12. Thepower transmitting coil L1, the power receiving coil L2, and theauxiliary coil L3 may be configured such that the coupling coefficientk23 is smaller than the coupling coefficient k13.

Operation Example of First Embodiment

FIG. 3 is a diagram showing an application example of the contactlesspower transmission system shown in FIG. 1. FIG. 3 shows a case where thepower receiver apparatus 20 is built into an electrically-driven vehicle32 with a battery, and the power transmitter apparatus 10 is built intoa road surface 31 so as to be able to transmit power to the powerreceiver apparatus 20 of the vehicle 32. In this case, the battery ofthe vehicle 32 corresponds to the load device 22 of the power receiverapparatus 20. The power transmitter apparatus 10 and the power receiverapparatus 20 oppose to each other at a distance dl. As shown in FIG. 3,a foreign object 33 may appear between the power transmitting coil L1and the power receiving coil L2.

FIG. 4 is a graph showing an example of a change in magnitude of thecurrent I3 that is generated by the auxiliary coil L3 and detected bythe detector 13 shown in FIG. 1. FIG. 5 is a graph showing an example ofa change in magnitude of the current I1 flowing through the powertransmitting coil L1 and detected by the detector 14 shown in FIG. 1. Asdescribed above, the coupling coefficient k12 between the powertransmitting coil L1 and the power receiving coil L2 varies depending onthe distance dl between the power transmitting coil L1 and the powerreceiving coil L2. Therefore, a relationship between the distance dl andthe currents I1 and I3 shown in FIGS. 4 and 5 equivalently indicates arelationship between the coupling coefficient k12 and the currents I1and I3. In addition, as described above, when the foreign object 33exists between the power transmitting coil L1 and the power receivingcoil L2, the currents I1 and I3 are differently affected by the foreignobject 33. In the examples shown in FIGS. 4 and 5, when the foreignobject 33 exists, the current I3 decreases and the current I1 increasesas compared to those of the case where no foreign object 33 exists.

FIG. 6 is a table showing examples of the coupling coefficient k12between the power transmitting coil L1 and the power receiving coil L2,the coupling coefficient k12 being calculated for the current I1 flowingthrough the power transmitting coil L1 and the current I3 generated bythe auxiliary coil L3 shown in FIG. 1. FIG. 6 shows the currents I1 andI3 and the coupling coefficient k12, when the power receiver apparatus20 is in the test mode, and no foreign object 33 exists between thepower transmitting coil L1 and the power receiving coil L2. The couplingcoefficient estimator 15 stores, in advance, the table indicating therelationship between the currents I1 and I3 and the coupling coefficientk12 as shown in FIG. 6. The coupling coefficient estimator 15 estimatesthe first coupling coefficient k12 a between the power transmitting coilL1 and the power receiving coil L2, by referring to the table based onthe value of the current I3. The coupling coefficient estimator 15estimates the second coupling coefficient k12 b between the powertransmitting coil L1 and the power receiving coil L2, by referring tothe table based on the value of the current I1. When no foreign object33 exists between the power transmitting coil L1 and the power receivingcoil L2, it is expected that the coupling coefficient k12 a estimatedbased on the value of the current I3 is equal to the couplingcoefficient k12 b estimated based on the value of the current I1. On theother hand, when the foreign object 33 exists between the powertransmitting coil L1 and the power receiving coil L2, the currents I1and I3 are differently affected by the foreign object 33, and as aresult, the coupling coefficient k12 a estimated based on the value ofthe current I3 is different from the coupling coefficient k12 bestimated based on the value of the current I1. Therefore, it ispossible to determine whether or not the foreign object 33 existsbetween the power transmitting coil L1 and the power receiving coil L2,based on whether or not the coupling coefficients k12 a and k12 b areequal to each other.

When the estimated coupling coefficients k12 a and k12 b aresubstantially equal to each other, that is, when the difference betweenthe coupling coefficients k12 a and k12 b is less than or equal to thepredetermined threshold, the control circuit 16 may determine that thecoupling coefficients k12 a and k12 b are equal to each other.

The coupling coefficient estimator 15 may store, in advance, formulasindicating the relationship between the currents I1 and I3 and thecoupling coefficient k12, instead of the table as shown in FIG. 6. Forexample, the coupling coefficient k12 a may be estimated based on thecurrent I1 flowing through the power transmitting coil L1, as follows.k12a=e ^(I1) +a

Where “a” on the right side is a constant.

In addition, the current I1 and the coupling coefficient k12 a may havethe following relationship.I1=1+k12a+(k12a)²+ . . . +(k12a)^(n)

By solving this equation for the coupling coefficient k12 a, thecoupling coefficient k12 a may be estimated based on the current I1.

The formulas for estimating the coupling coefficient k12 a based on thecurrent I1 are not limited to the examples given above.

Also in a case where the coupling coefficient k12 b is estimated basedon the current I3 flowing through the auxiliary coil L3, the couplingcoefficient k12 b can be estimated using some formula in a similarmanner as that of the case where the coupling coefficient k12 a isestimated based on the current I1.

FIGS. 4 and 6 shows the case of detecting the value of the current I3generated by the auxiliary coil L3. Also in the case of detecting thevalue of the voltage V3 generated by the auxiliary coil L3, the couplingcoefficient k12 b between the power transmitting coil L1 and the powerreceiving coil L2 can be estimated in a substantially similar manner.

FIG. 7 is a flowchart showing a first power transmission processexecuted by the control circuit 16 shown in FIG. 1.

In step S1, the control circuit 16 receives a signal from the powerreceiver apparatus 20 via the communication device 17, the signalindicating that the power receiver apparatus 20 has requested powertransmission.

In step S2, the control circuit 16 transmits a signal to the powerreceiver apparatus 20 via the communication device 17, the signalrequesting transition to the test mode. When receiving the signalrequesting transition to the test mode, from the power transmitterapparatus 10 via the communication device 24, the control circuit 23 ofthe power receiver apparatus 20 controls the switch circuit SW to supplythe voltage V4 outputted from the rectifier circuit 21, to the loadelement R2. In step S3, the control circuit 16 sets the voltage V0outputted from the AC/DC converter 11, and the switching frequency fswof the inverter 12, to predetermined values for the test mode, andstarts power transmission in the test mode. As described above, thevoltage generated in the power receiver apparatus 20 varies depending onthe coupling coefficient k12 between the power transmitting coil L1 andthe power receiving coil L2, but the coupling coefficient k12 is unknownuntil completion of the test mode. Therefore, in order to preventovervoltage in the power receiver apparatus 20, the control circuit 16sets the voltage V0 outputted from the AC/DC converter 11, to apredetermined non-zero minimum, and sets the switching frequency fsw ofthe inverter 12 to the maximum. The minimum of the voltage V0 is setsuch that the currents I1 and I3 (alternatively, the current I1 and thevoltage V3), with which the coupling coefficients k12 a and k12 b can bedetected, are generated by the power transmitting coil L1 and theauxiliary coil L3. The minimum of the voltage V0 and the maximum of theswitching frequency fsw are used as the predetermined values for thetest mode.

In step S4, the control circuit 16 detects the value of the current I3or voltage V3 generated by the auxiliary coil L3, using the detector 13.In step S5, the control circuit 16 estimates the coupling coefficientk12 a between the power transmitting coil L1 and the power receivingcoil L2, using the coupling coefficient estimator 15, by referring tothe table or formulas based on the value of the detected current I3 orvoltage V3.

In step S6, the control circuit 16 detects the value of the current I1flowing through the power transmitting coil L1, using the detector 14.In step S7, the control circuit 16 estimates the coupling coefficientk12 b between the power transmitting coil L1 and the power receivingcoil L2, using the coupling coefficient estimator 15, by referring tothe table or formulas based on the value of the detected current I1.

In step S8, the control circuit 16 determines whether or not theestimated coupling coefficients k12 a and k12 b are substantially equalto each other; if YES, the process proceeds to step S9, and if NO, theprocess proceeds to step S12.

In step S9, the control circuit 16 sets the voltage V0 outputted fromthe AC/DC converter 11, and the switching frequency fsw of the inverter12, to predetermined values for the normal mode. The control circuit 16sets the voltage V0 outputted from the AC/DC converter 11, to a valuepredetermined in accordance with the value of the voltage and/or currentto be supplied to the load device 22. The control circuit 16 sets theswitching frequency fsw of the inverter 12, for example, to atransmitting frequency at least locally minimizing the dependence of theoutput voltage of the power receiver apparatus 20 on the load value ofthe power receiver apparatus 20, as described in the second embodiment.Such values of the voltage V0 and switching frequency fsw are used asthe predetermined values for the normal mode. In step S10, the controlcircuit 16 transmits a signal to the power receiver apparatus 20 via thecommunication device 17, the signal requesting transition to the normalmode. When receiving the signal requesting transition to the normalmode, from the power transmitter apparatus 10 via the communicationdevice 24, the control circuit 23 of the power receiver apparatus 20controls the switch circuit SW to supply the voltage V4 outputted fromthe rectifier circuit 21, to the load device 22. In step S11, thecontrol circuit 16 starts power transmission in the normal mode.

In step S12, the control circuit 16 determines that a foreign objectexists. In step S13, the control circuit 16 stops transmitting power.

[Advantageous Effects of First Embodiment]

According to the contactless power transmission system of the firstembodiment, it is possible to surely detect a foreign object through asimple configuration without a camera, a temperature sensor, or thelike, and without depending on feedback from the power receiverapparatus 20 to the power transmitter apparatus 10.

According to the contactless power transmission system of the firstembodiment, it is possible to stop power transmission when detecting aforeign object, thus improving the safety of the contactless powertransmission system.

According to the contactless power transmission system of the firstembodiment, when the coupling coefficient k12 a estimated based on thevalue of the current I3 or voltage V3 is equal to the couplingcoefficient k12 b estimated based on the value of the current I1, it ispossible to accurately estimate the coupling coefficient k12 between thepower transmitting coil L1 and the power receiving coil L2.

In the case of estimating the coupling coefficient between the powertransmitting coil and the power receiving coil based on one circuitparameter, and comparing the estimated coupling coefficient with somethreshold, it is difficult to distinguish whether the couplingcoefficient has varied due to the influence of the foreign object or dueto other factors (such as a change in distance between the powertransmitting coil and the power receiving coil). In addition, in thiscase, it is only possible to determine whether the estimated couplingcoefficient is higher or lower than the threshold, and therefore, it isnot possible to take the strength of the coupling coefficient intoconsideration. On the other hand, according to the contactless powertransmission system of the first embodiment, it is possible to surelydetect a foreign object regardless of whether the coupling coefficientsare high or low, by determining whether or not the two estimatedcoupling coefficients k12 a and k12 b are substantially equal to eachother.

Second Embodiment

A contactless power transmission system according to a second embodimentwill be described with reference to FIG. 8 to FIG. 12.

Application Example of Second Embodiment

As described above, when the power transmitter apparatus transmits powerto the power receiver apparatus in a contactless manner, the powerreceiver apparatus is not always placed at a fixed position relative tothe power transmitter apparatus. Therefore, a distance between q powertransmitting coil of the power transmitter apparatus and q powerreceiving coil of the power receiver apparatus may vary, and a couplingcoefficient between the power transmitting coil and the power receivingcoil may vary accordingly. When the coupling coefficient between thepower transmitting coil and the power receiving coil varies, a voltageand/or current supplied from the power receiver apparatus to a loaddevice varies accordingly.

In order for the power receiver apparatus to supply the load device witha desired voltage thereof, it may be possible to, for example, feed anoutput voltage value and/or output current value of the power receiverapparatus back to the power transmitter apparatus, and regulate avoltage applied to the power transmitting coil. However, in this case,in order to feed back the output voltage value and/or output currentvalue of the power receiver apparatus to the power transmitterapparatus, some delay occurs, and it is difficult to follow rapidvariations in a load value of the power receiver apparatus. In addition,in order for the power receiver apparatus to supply the load device witha desired voltage thereof, it may be possible to, for example, providethe power receiver apparatus with a DC/DC converter. However, in thiscase, the size, weight, and cost of the power receiver apparatusincrease. Therefore, it is required to control the power transmitterapparatus to supply the load device with a desired voltage thereof,without depending on the feedback from the power receiver apparatus tothe power transmitter apparatus, and without providing the powerreceiver apparatus with extra circuits (DC/DC converter or the like).

According to the second embodiment, a contactless power transmissionsystem will be described, which is capable of controlling only the powertransmitter apparatus based only on information that can be acquired bythe power transmitter apparatus, to supply a load device with a desiredvoltage thereof.

The contactless power transmission system according to the secondembodiment is configured in a substantially similar manner as that ofthe contactless power transmission system according to the firstembodiment. Hereinafter, the contactless power transmission systemaccording to the second embodiment will be described with reference toFIG. 1.

The contactless power transmission system according to the secondembodiment includes the power transmitter apparatus 10 and the powerreceiver apparatus 20, and the power transmitter apparatus 10 transmitspower to the power receiver apparatus 20 in a contactless manner.

The power transmitter apparatus 10 is provided with at least the AC/DCconverter 11, the inverter 12, detectors 13 and 14, the control circuit16, the power transmitting coil L1, and the auxiliary coil L3.

The AC/DC converter 11 converts the AC voltage inputted from the ACpower source 1, into the DC voltage V0 having a magnitude variable underthe control of the control circuit 16. The inverter 12 operates at theswitching frequency fsw variable under the control of the controlcircuit 16, and converts the DC voltage V0 inputted from the AC/DCconverter 11, into the AC voltage V1. The voltage V1 is applied acrossthe power transmitting coil L1. The amplitude of the voltage V1 is equalto the magnitude of the voltage V0. In other words, the AC/DC converter11 and the inverter 12 (power supply circuit) generate transmittingpower having a variable voltage and a variable frequency, and supply thetransmitting power to the power transmitting coil L1.

The power transmitting coil L1 and the auxiliary coil L3 of thecontactless power transmission system according to the second embodimentare configured in a similar manner as that of the power transmittingcoil L1 and the auxiliary coil L3 of the contactless power transmissionsystem according to the first embodiment, respectively.

The detectors 13 and 14 (detection circuit) detect at least one of thevalue of the current flowing through the power transmitting coil L1, andthe value of the current or voltage generated by the auxiliary coil L3.The values detected by the detectors 13 and 14 are notified to thecontrol circuit 16.

The control circuit 16 regulates the magnitude of the voltage V0outputted from the AC/DC converter 11, and the switching frequency fswof the inverter 12.

The power receiver apparatus 20 of the contactless power transmissionsystem according to the second embodiment is configured in a similarmanner as that of the power receiver apparatus 20 of the contactlesspower transmission system according to the first embodiment.

The control circuit 16 of the power transmitter apparatus 10 determinesa transmitting frequency based on the values detected by the detectors13 and 14 (detection circuit), the transmitting frequency at leastlocally minimizing the dependence of the output voltage of the powerreceiver apparatus 20 on the load value of the power receiver apparatus20. In the present specification, such a frequency is also referred toas a “load-independent transmitting frequency”. The control circuit 16determines a voltage for the transmitting power at which the outputvoltage of the power receiver apparatus 20 is equal to a predeterminedtarget voltage when generating the transmitting power having thedetermined transmitting frequency. The control circuit 16 controls theAC/DC converter 11 and the inverter 12 (power supply circuit) togenerate the transmitting power having the determined frequency and thedetermined voltage.

According to the second embodiment, the detectors 13 and 14 and thecontrol circuit 16 are collectively referred to as a “control apparatus”of the power transmitter apparatus 10.

The voltage V4 applied to the load device 22 (output voltage of thepower receiver apparatus 20) varies depending on the couplingcoefficient k12 between the power transmitting coil L1 and the powerreceiving coil L2. The voltage V4 applied to the load device 22 alsovaries depending on the load value of the load device 22. The voltage V4decreases as the load value increases, and the voltage V4 increases asthe load value decreases. Even under such conditions, it is required tocontrol the power transmitter apparatus 10 to supply the load device 22with a desired voltage thereof.

The contactless power transmission system according to the secondembodiment determines a transmitting frequency based on the valuesdetected by the detectors 13 and 14, the transmitting frequency at leastlocally minimizing the dependence of the output voltage of the powerreceiver apparatus 20 on the load value of the power receiver apparatus20, as described above. In addition, the contactless power transmissionsystem according to the second embodiment determines the voltage for thetransmitting power at which the output voltage of the power receiverapparatus 20 is equal to the predetermined target voltage (that is, adesired voltage of the load device 22) when generating the transmittingpower having the determined transmitting frequency, as described above.Here, the “voltage for the transmitting power” means the amplitude oftransmitting power. In addition, the contactless power transmissionsystem according to the second embodiment controls the AC/DC converter11 and the inverter 12 to generate the transmitting power having thedetermined frequency and the determined voltage, as described above.Thus, by controlling only the power transmitter apparatus 10 based onlyon the information that can be acquired by the power transmitterapparatus 10, it is possible to supply the load device 22 with a desiredvoltage thereof, substantially without depending on the load value ofthe power receiver apparatus 20 (the magnitude of the current flowingthrough the load device 22). It is not necessary to monitor variationsin the load value of the power receiver apparatus 20, thus eliminatingthe need for feedback from the power receiver apparatus 20 to the powertransmitter apparatus 10, and facilitating to follow rapid variations inthe load value. In addition, by eliminating the need for providing thepower receiver apparatus with extra circuits (DC/DC converter or thelike), it is possible to reduce the size, weight, and cost of the powerreceiver apparatus 20, or at least, make the size, weight, and cost lesslikely to increase.

According to the second embodiment, the power receiver apparatus 20 maybe an electronic device with a battery, and the power transmitterapparatus 10 may be a charger for the power receiver apparatus 20, in amanner similar to that of the first embodiment. In addition, accordingto the second embodiment, the power receiver apparatus 20 may be anelectrically-driven vehicle with a battery, and the power transmitterapparatus 10 may a charging stand for the power receiver apparatus 20,in a manner similar to that of the first embodiment. In addition,according to the second embodiment, the power receiver apparatus 20 maybe a pallet that requires a power source, and the power transmitterapparatus 10 may be a conveyor capable of supplying power to suchpallets, in a manner similar to that of the first embodiment. Inaddition, according to the second embodiment, the power transmitterapparatus 10 and the power receiver apparatus 20 may be provided insteadof a slip ring, at a joint of a robot arm or the like, in a mannersimilar to that of the first embodiment.

Configuration Example of Second Embodiment

As described above, the contactless power transmission system accordingto the second embodiment is configured in a substantially similar manneras that of the contactless power transmission system according to thefirst embodiment. However, the control circuit 16 of the powertransmitter apparatus 10 is configured to execute a second powertransmission process to be described later with reference to FIG. 8,instead of the first power transmission process shown in FIG. 7.Therefore, the control circuit 16 stores, in advance, a table orformulas as described below.

The control circuit 16 stores, in advance, a table or formulasindicating a relationship between the coupling coefficient k12 betweenthe power transmitting coil L1 and the power receiving coil L2, and theload-independent transmitting frequency. Alternatively, the couplingcoefficient estimator 15 may be omitted, and the values detected by thedetectors 13 and 14 may be directly inputted to the control circuit 16.In this case, the control circuit 16 may store, in advance, a table orformulas indicating a relationship between at least one of the value ofthe current flowing through the power transmitting coil L1 and the valueof the current or voltage generated by the auxiliary coil L3, and theload-independent transmitting frequency. By referring to the table orformulas. the control circuit 16 can determine a transmitting frequency(that is, the switching frequency fsw of the inverter 12) at leastlocally minimizing the dependence of the output voltage of the powerreceiver apparatus 20 on the load value of the power receiver apparatus20.

The control circuit 16 further stores, in advance, a table or formulasindicating a relationship between the voltage V0 outputted from theAC/DC converter 11 and the output voltage of the power receiverapparatus 20, for various coupling coefficients k12 between the powertransmitting coil L1 and the power receiving coil L2. By referring tothe table or formulas, the control circuit 16 can determine the voltagefor the transmitting power at which the output voltage of the powerreceiver apparatus 20 is equal to the predetermined target voltage.

By referring to these tables or formulas, the control circuit 16executes a second power transmission process (or a third or fourth powertransmission process) to be described later.

In addition, the control circuit 16 may transmit a signal to the powerreceiver apparatus 20 using the communication device 17 the signalrequesting transition to the test mode. The control circuit 16 maydetermine the load-independent transmitting frequency based on thevalues detected by the detectors 13 and 14 when the power receiverapparatus 20 is in the test mode. Thus, it is possible to correctlydetermine the load-independent transmitting frequency, even when theload device 22 has a variable load value. Then, by controlling only thepower transmitter apparatus 10 based only on the information that can beacquired by the power transmitter apparatus 10, it is possible to supplythe load device 22 with a desired voltage thereof, substantially withoutdepending on the load value of the power receiver apparatus 20 (themagnitude of the current flowing through the load device 22).

The control circuit 16 may determine the load-independent transmittingfrequency based on the coupling coefficient k12 between the powertransmitting coil L1 and the power receiving coil L2 estimated by thecoupling coefficient estimator 15.

The coupling coefficient estimator 15 may estimate the first couplingcoefficient k12 a between the power transmitting coil L1 and the powerreceiving coil L2, based on the value of the current or voltagegenerated by the auxiliary coil L3, and estimate the second couplingcoefficient k12 b between the power transmitting coil L1 and the powerreceiving coil L2, based on the value of the current flowing through thepower transmitting coil L1. When the first coupling coefficient k12 aand the second coupling coefficient k12 b are substantially equal toeach other, the control circuit 16 may control the AC/DC converter 11and the inverter 12 to generate transmitting power having the determinedfrequency and the determined voltage. In addition, when the firstcoupling coefficient k12 a and the second coupling coefficient k12 b aredifferent from each other, the control circuit 16 may control the AC/DCconverter 11 and the inverter 12 to stop power transmission to the powerreceiver apparatus 20. Thus, the control circuit 16 can correctly andsurely determine whether or not a foreign object exists between thepower transmitting coil L1 and the power receiving coil L2, regardlessof a change in the coupling coefficient between the power transmittingcoil L1 and the power receiving coil L2.

Operation Example of Second Embodiment

FIG. 8 is a flowchart showing a second power transmission processexecuted by the control circuit 16 of the contactless power transmissionsystem according to the second embodiment. In the second powertransmission process shown in FIG. 8, the control circuit 16 executessteps S21 and S22, instead of step S9 shown in FIG. 7.

In step S21, the control circuit 16 sets the switching frequency fsw ofthe inverter 12 based on the coupling coefficient k12 (equal to k12 aand k12 b) between the power transmitting coil L1 and the powerreceiving coil L2. In step S22, the control circuit 16 sets the voltageV0 outputted from the AC/DC converter 11 based on the voltage V4 to beapplied to the load device 22.

FIG. 9 is an exemplary graph illustrating that the output voltage of thepower receiver apparatus 20 shown in FIG. 1 varies depending on thecoupling coefficient k12 between the power transmitting coil L1 and thepower receiving coil L2, and depending on the voltage V0 of the powertransmitter apparatus 10. In the case of a large distance dl between thepower transmitting coil L1 and the power receiving coil L2, and a lowcoupling coefficient k12, the voltage V4 is at a local maximum when theswitching frequency fsw is set to f1. In the case of a small distance dlbetween the power transmitting coil L1 and the power receiving coil L2,and a high coupling coefficient k12, the voltage V4 is at a localmaximum when the switching frequency fsw is set to f2. Here, the word“large” or “small” of the distance dl, and the word “high” or “low” ofthe coupling coefficient k12 mean a relative magnitude thereof. Thecontrol circuit 16 sets the switching frequency fsw of the inverter 12in accordance with the load-independent transmitting frequency. When thevoltage V4 applied to the load device 22 is less than the targetvoltage, the control circuit 16 increases the voltage V0 outputted fromthe AC/DC converter 11 to increase the voltage V4 to the target voltage.When the voltage V4 applied to the load device 22 is greater than thetarget voltage, the control circuit 16 reduces the voltage V0 outputtedfrom the AC/DC converter 11 to reduce the voltage V4 to the targetvoltage.

FIG. 10 is an exemplary graph illustrating that the output voltage ofthe power receiver apparatus 20 shown in FIG. 1 varies depending on thecoupling coefficient k12 between the power transmitting coil L1 and thepower receiving coil L2, and depending on the load value of the loaddevice 22. FIG. 10 shows a case where a constant voltage V0 is outputtedfrom the AC/DC converter 11. For example, when the load device 22 is abattery, the load device 22 has a variable load value that variesdepending on a state of charge of the battery. As described above, thevoltage V4 applied to the load device 22 varies depending on the loadvalue of the load device 22. However, as shown in FIG. 10, whentransmitting power having a certain switching frequency fsw, thedependence of the voltage V4 on the load value is at least locallyminimized, and the voltage V4 becomes substantially constant regardlessof the load value of the load device 22. The load-independenttransmitting frequency may be equal to or different from the switchingfrequency fsw at which the voltage V4 is maximized. Therefore, bysetting the switching frequency fsw of the inverter 12 in accordancewith such a load-independent transmitting frequency, it is not necessaryto control the power transmitter apparatus 10 and/or the power receiverapparatus 20 in accordance with the load value of the load device 22.Thus, it is not necessary to monitor the load value of the load device22 and feed back the load value from the power receiver apparatus 20 tothe power transmitter apparatus 10, and also not necessary to providethe power receiver apparatus 20 with extra circuits (DC/DC converter orthe like) in order to supply the load device 22 with a desired voltagethereof.

First Modified Embodiment of Second Embodiment

FIG. 11 is a flowchart showing a third power transmission processexecuted by the control circuit 16 of the contactless power transmissionsystem according to a first modified embodiment of the secondembodiment. When it is assumed that no foreign object exists between thepower transmitting coil L1 and the power receiving coil L2, steps S6 toS8, S12, and S13 shown in FIG. 8 may be omitted. In this case, thecurrent detection resistor R1 and the detector 14 shown in FIG. 1 may beomitted. Thus, it is possible to simplify the configuration andoperation of the power transmitter apparatus 10, as compared with thoseof FIG. 1 and FIG. 8.

Second Modified Embodiment of Second Embodiment

FIG. 12 is a flowchart showing a fourth power transmission processexecuted by the control circuit 16 of the contactless power transmissionsystem according to a second modified embodiment of the secondembodiment. When it is assumed that no foreign object exists between thepower transmitting coil L1 and the power receiving coil L2, steps S4,S5, S8, S12, and S13 shown in FIG. 8 may be omitted. In this case, theauxiliary coil L3 and the detector 13 shown in FIG. 1 may be omitted.Thus, it is possible to simplify the configuration and operation of thepower transmitter apparatus 10, as compared with those of FIGS. 1 and 8.

Advantageous Effects of Second Embodiment

According to the contactless power transmission system of the secondembodiment, it is possible to supply the load device 22 with a desiredvoltage thereof, substantially without depending on the load value ofthe power receiver apparatus 20, by controlling only the powertransmitter apparatus 10 based only on the information that can beacquired by the power transmitter apparatus 10.

According to the contactless power transmission system of the secondembodiment, it is possible to surely detect a foreign object through asimple configuration without a camera, a temperature sensor, or thelike, and without depending on feedback from the power receiverapparatus 20 to the power transmitter apparatus 10.

According to the contactless power transmission system of the secondembodiment, it is possible to stop power transmission when detecting aforeign object, thus improving the safety of the contactless powertransmission system.

Other Modified Embodiments

Although the embodiments of the present disclosure have been describedin detail above, the above descriptions are merely examples of thepresent disclosure in all respects. Needless to say, variousimprovements and modifications can be made without departing from thescope of the present disclosure. For example, the following changes canbe made. Hereinafter, components similar to those of the aboveembodiments are indicated by similar reference signs, and points similarto those of the above embodiments will be omitted as appropriate.

The above-described embodiments and modifications may be combined in anymanner.

The embodiments described herein are merely examples of the presentdisclosure in all respects. Needless to say, various improvements ormodifications can be made without departing from the scope of thepresent disclosure. That is, in order to implement the presentdisclosure, a specific configuration according to the embodiments may beemployed as needed.

The power transmitter apparatus may use a DC power source, instead ofthe AC power source. In this case, the power transmitter apparatus maybe provided with a DC/DC converter, instead of the AC/DC converter.

The power transmitter apparatus may detect the power receiver apparatususing any sensor or switch other than the communication device.

FIG. 1 shows the case where the power transmitting coil L1 and thecapacitor C1 are connected in series, and the power receiving coil L2and the capacitor C2 are connected in series. However, at least one ofthese may be connected parallel.

The power transmitting coil, the power receiving coil, and the auxiliarycoil may have a shape other than the ring shown in FIG. 2.

In order to detect the current I1 flowing through the power transmittingcoil L1, for example, a shunt resistor, a current transformer, or thelike may be used instead of the current detection resistor R1.

The load device may be integrated inside the power receiver apparatus asshown in FIG. 1, or may be connected externally to the power receiverapparatus.

The load device 22 may have a predetermined load value, instead of avariable load value.

SUMMARY OF EMBODIMENTS

The control apparatus for the power transmitter apparatus, the powertransmitter apparatus, and the contactless power transmission systemaccording to aspects of the present disclosure may be expressed asfollows.

According to a control apparatus for a power transmitter apparatus (10)of a first aspect of the present disclosure, the control apparatus forthe power transmitter apparatus (10) transmits power to a power receiverapparatus (20) provided with a power receiving coil (L2), in acontactless manner. The power transmitter apparatus (10) is providedwith: a power transmitting coil (L1), an auxiliary coil (L3)electromagnetically coupled to the power transmitting coil (L1), and apower supply circuit (11, 12) that generates transmitting power having avoltage and a frequency and supplies the transmitting power to the powertransmitting coil (L1). The control apparatus is provided with: a firstdetection circuit (13, 14), a second detection circuit (13, 14), acoupling coefficient estimator (15), and a control circuit (16). Thefirst detection circuit (13, 14) detects a value of a current or voltagegenerated by the auxiliary coil (L3). The second detection circuit (13,14) detects a value of a current flowing through the power transmittingcoil (L1). The coupling coefficient estimator (15) estimates a firstcoupling coefficient between the power transmitting coil (L1) and thepower receiving coil (L2), based on the value of the current or voltagegenerated by the auxiliary coil (L3), and estimates a second couplingcoefficient between the power transmitting coil (L1) and the powerreceiving coil (L2), based on the value of the current flowing throughthe power transmitting coil (L1). The control circuit (16) controls thepower supply circuit (11, 12) to transmit power to the power receiverapparatus (20) when a difference between the first and second couplingcoefficients is equal to or less than a predetermined threshold, andstop power transmission to the power receiver apparatus (20) when thedifference between the first and second coupling coefficients is greaterthan the threshold.

According to the control apparatus for the power transmitter apparatus(10) of a second aspect of the present disclosure, in the controlapparatus for the power transmitter apparatus (10) of the first aspectof the present disclosure, the power receiver apparatus (20) is providedwith a first load device (22) having a variable load value, a secondload device (R2) having a predetermined load value, and a switch circuit(SW) that selectively supplies the output voltage of the power receiverapparatus (20) to one of the first load device (22) and the second loaddevice (R2). The control apparatus is further provided with acommunication device (17) communicatively connected to the powerreceiver apparatus (20). When transmitting power in a normal manner, thecontrol circuit (16) transmits a signal to the power receiver apparatus(20) using the communication device (17), the signal controlling theswitch circuit (SW) to supply the output voltage of the power receiverapparatus (20) to the first load device (22). When estimating the firstand second coupling coefficients, the control circuit (16) transmits asignal to the power receiver apparatus (20) using the communicationdevice (17), the signal controlling the switch circuit (SW) to supplythe output voltage of the power receiver apparatus (20) to the secondload device (R2), and estimates the first and second couplingcoefficients based on the values detected by the first and seconddetectors (13, 14).

According to a power transmitter apparatus (10) of a third aspect of thepresent disclosure, the power transmitter apparatus (10) is providedwith: a power transmitting coil (L1); an auxiliary coil (L3)electromagnetically coupled to the power transmitting coil (L1); a powersupply circuit (11, 12) that generates transmitting power having avoltage and a frequency and supplies the transmitting power to the powertransmitting coil (L1); and the control apparatus for the powertransmitter apparatus (10) of the first or second aspect of the presentdisclosure.

According to the power transmitter apparatus (10) of a fourth aspect ofthe present disclosure, in the power transmitter apparatus (10) of thethird aspect of the present disclosure, the power transmitter apparatus(10) is further provided with a capacitor (C1) connected to the powertransmitting coil (L1) so as to form an LC resonant circuit.

According to the power transmitter apparatus (10) of a fifth aspect ofthe present disclosure, in the power transmitter apparatus (10) of thethird or fourth aspect of the present disclosure, the power transmitterapparatus (10) is further provided with a magnetic core (F1) aroundwhich the power transmitting coil (L1) and the auxiliary coil (L3) arewound. The auxiliary coil (L3) is disposed so as to surround the powertransmitting coil (L1).

According to a contactless power transmission system of a sixth aspectof the present disclosure, the contactless power transmission systemincludes: the power transmitter apparatus (10) of any one of the firstto fifth aspects of the present disclosure; and a power receiverapparatus (20) provided with a power receiving coil (L2).

According to a control apparatus for a power transmitter apparatus (10)of a seventh aspect of the present disclosure, the control apparatus fora power transmitter apparatus (10) transmits power to a power receiverapparatus (20) provided with a power receiving coil (L2), in acontactless manner. The power transmitter apparatus (10) is providedwith: a power transmitting coil (L1), an auxiliary coil (L3)electromagnetically coupled to the power transmitting coil (L1), and apower supply circuit (11, 12) that generates transmitting power having avariable voltage and a variable frequency and supplies the transmittingpower to the power transmitting coil (L1). The control apparatus isprovided with a detection circuit (13, 14) and a control circuit (16).The detection circuit (13, 14) detects at least one of a value of acurrent flowing through the power transmitting coil (L1), and a value ofa current or voltage generated by the auxiliary coil (L3). The controlcircuit (16) determines a transmitting frequency based on the valuedetected by the detection circuit (13, 14), the transmitting frequencyat least locally minimizing dependence of an output voltage of the powerreceiver apparatus (20) on a load value of the power receiver apparatus(20), determines a voltage for the transmitting power at which theoutput voltage of the power receiver apparatus (20) is equal to apredetermined target voltage when generating the transmitting powerhaving the transmitting frequency determined, and controls the powersupply circuit (11, 12) to generate the transmitting power having thetransmitting frequency and voltage determined.

According to the control apparatus for the power transmitter apparatus(10) of an eighth aspect of the present disclosure, in the controlapparatus for the power transmitter apparatus (10) of the seventh aspectof the present disclosure, the power receiver apparatus (20) is providedwith a first load device (22) having a variable load value, a secondload device (R2) having a predetermined load value, and a switch circuit(SW) that selectively supplies the output voltage of the power receiverapparatus (20) to one of the first load device (22) and the second loaddevice (R2). The control apparatus is further provided with acommunication device (17) communicatively connected to the powerreceiver apparatus (20). When transmitting power in a normal manner, thecontrol circuit (16) transmits a signal to the power receiver apparatus(20) using the communication device (17), the signal controlling theswitch circuit (SW) to supply the output voltage of the power receiverapparatus (20) to the first load device (22). When determining thetransmitting frequency, the control circuit (16) transmits a signal tothe power receiver apparatus (20) using the communication device (17),the signal controlling the switch circuit (SW) to supply the outputvoltage of the power receiver apparatus (20) to the second load device(R2), and determines the transmitting frequency based on the valuedetected by the detection circuit (13, 14).

According to the control apparatus for the power transmitter apparatus(10) of a ninth aspect of the present disclosure, in the controlapparatus for the power transmitter apparatus (10) of the seventh oreighth aspect of the present disclosure, the control apparatus isfurther provided with a coupling coefficient estimator (15) thatestimates a coupling coefficient between the power transmitting coil(L1) and the power receiving coil (L2) based on the value detected bythe detection circuit (13, 14), and determines the transmittingfrequency based on the coupling coefficient.

According to the control apparatus for the power transmitter apparatus(10) of a tenth aspect of the present disclosure, in the controlapparatus for the power transmitter apparatus (10) of the ninth aspectof the present disclosure, the detection circuit (13, 14) is providedwith a first detector (13) that detects the value of the current orvoltage generated by the auxiliary coil (L3), and a second detector (14)that detects the current flowing through the power transmitting coil(L1). The coupling coefficient estimator (15) estimates a first couplingcoefficient between the power transmitting coil (L1) and the powerreceiving coil (L2) based on the value of the current or voltagegenerated by the auxiliary coil (L3), and estimates a second couplingcoefficient between the power transmitting coil (L1) and the powerreceiving coil (L2) based on the value of the current flowing throughthe power transmitting coil (L1). When a difference between the firstand second coupling coefficients is equal to or less than apredetermined threshold, the control circuit (16) controls the powersupply circuit (11, 12) to generate the transmitting power having thefrequency and voltage determined.

According to the control apparatus for the power transmitter apparatus(10) of an eleventh aspect of the present disclosure, in the controlapparatus for the power transmitter apparatus (10) of the tenth aspectof the present disclosure, when the difference between the first andsecond coupling coefficients is greater than the predeterminedthreshold, the control circuit (16) controls the power supply circuit(11, 12) to stop power transmission to the power receiver apparatus(20).

According to a power transmitter apparatus (10) of a twelfth aspect ofthe present disclosure, the power transmitter apparatus (10) is providedwith: a power transmitting coil (L1); an auxiliary coil (L3)electromagnetically coupled to the power transmitting coil (L1); a powersupply circuit (11, 12) that generates transmitting power having avariable voltage and a variable frequency and supplies the transmittingpower to the power transmitting coil (L1); and a control apparatus forthe power transmitter apparatus (10) of any one of the seventh toeleventh aspects of the present disclosure.

According to the power transmitter apparatus (10) of a thirteenth aspectof the present disclosure, in the power transmitter apparatus (10) ofthe twelfth aspect of the present disclosure, the power transmitterapparatus (10) is further provided with a capacitor (C1) connected tothe power transmitting coil (L1) so as to form an LC resonant circuit.

According to the power transmitter apparatus (10) of a fourteenth aspectof the present disclosure, in the power transmitter apparatus (10) ofthe twelfth or thirteenth aspect of the present disclosure, the powertransmitter apparatus (10) is further provided with a magnetic core (F1)around which the power transmitting coil (L1) and the auxiliary coil(L3) are wound. The auxiliary coil (L3) is disposed so as to surroundthe power transmitting coil (L1).

According to a contactless power transmission system of a fifteenthaspect of the present disclosure, the contactless power transmissionsystem includes: the power transmitter apparatus (10) of any one of thetwelfth to fourteenth aspects of the present disclosure; and a powerreceiver apparatus (20) provided with a power receiving coil (L2).

INDUSTRIAL APPLICABILITY

The present disclosure is applicable to a contactless power transmissionsystem in which power is transmitted through a magnetic field, and thecoupling coefficient between the power transmitting coil and the powerreceiving coil may vary. The present disclosure is also applicable to acontactless power transmission system in which power is transmittedthrough a magnetic field, and the coupling coefficient between the powertransmitting coil and the power receiving coil is constant.

The invention claimed is:
 1. A control apparatus for a power transmitter apparatus that transmits power to a power receiver apparatus comprising a power receiving coil, in a contactless manner, wherein the power transmitter apparatus comprises: a power transmitting coil, an auxiliary coil electromagnetically coupled to the power transmitting coil, and a power supply circuit that generates transmitting power having a variable voltage and a variable frequency and supplies the transmitting power to the power transmitting coil, wherein the control apparatus comprises: a detection circuit that detects at least one value of a value of a current flowing through the power transmitting coil, and a value of a current or voltage generated by the auxiliary coil; and a control circuit that determines a transmitting frequency based on the at least one value detected by the detection circuit, the transmitting frequency at least locally minimizing dependence of an output voltage of the power receiver apparatus on a load value of the power receiver apparatus, determines a voltage for the transmitting power at which the output voltage of the power receiver apparatus is equal to a predetermined target voltage when generating the transmitting power having the determined transmitting frequency, and controls the power supply circuit to generate the transmitting power having the determined transmitting frequency and voltage.
 2. The control apparatus for the power transmitter apparatus according to claim 1, wherein the power receiver apparatus comprises a first load device having a variable load value, a second load device having a predetermined load value, and a switch circuit that selectively supplies the output voltage of the power receiver apparatus to one of the first load device and the second load device, wherein the control apparatus further comprises a communication circuit communicatively connected to the power receiver apparatus, wherein, when transmitting power in a normal manner, the control circuit transmits a signal to the power receiver apparatus using the communication circuit, the signal controlling the switch circuit to supply the output voltage of the power receiver apparatus to the first load device, and wherein, when determining the transmitting frequency, the control circuit transmits a second signal to the power receiver apparatus using the communication circuit, the second signal controlling the switch circuit to supply the output voltage of the power receiver apparatus to the second load device, and determines the transmitting frequency based on the value detected by the detection circuit.
 3. The control apparatus for the power transmitter apparatus according to claim 1, further comprising a coupling coefficient estimator that estimates a coupling coefficient between the power transmitting coil and the power receiving coil based on the at least one value detected by the detection circuit, and determines the transmitting frequency based on the coupling coefficient.
 4. The control apparatus for the power transmitter apparatus according to claim 3, wherein the detection circuit comprises a first detector that detects the value of the current or voltage generated by the auxiliary coil, and a second detector that detects the current flowing through the power transmitting coil, wherein the coupling coefficient estimator estimates a first coupling coefficient between the power transmitting coil and the power receiving coil based on the value of the current or voltage generated by the auxiliary coil, and estimates a second coupling coefficient between the power transmitting coil and the power receiving coil based on the value of the current flowing through the power transmitting coil, and wherein, when a difference between the first and second coupling coefficients is equal to or less than a predetermined threshold, the control circuit controls the power supply circuit to generate the transmitting power having the determined frequency and voltage.
 5. The control apparatus for the power transmitter apparatus according to claim 4, wherein, when the difference between the first and second coupling coefficients is greater than the predetermined threshold, the control circuit controls the power supply circuit to stop the power transmission to the power receiver apparatus.
 6. A power transmitter apparatus comprising: a power transmitting coil; an auxiliary coil electromagnetically coupled to the power transmitting coil; a power supply circuit that generates transmitting power having a variable voltage and a variable frequency and supplies the transmitting power to the power transmitting coil; and a control apparatus for the power transmitter apparatus, wherein the power transmitter apparatus transmits power to a power receiver apparatus comprising a power receiving coil, in a contactless manner, wherein the control apparatus comprises: a detection circuit that detects at least one value of a value of a current flowing through the power transmitting coil, and a value of a current or voltage generated by the auxiliary coil; and a control circuit that determines a transmitting frequency based on the at least one value detected by the detection circuit, the transmitting frequency at least locally minimizing dependence of an output voltage of the power receiver apparatus on a load value of the power receiver apparatus, determines a voltage for the transmitting power at which the output voltage of the power receiver apparatus is equal to a predetermined target voltage when generating the transmitting power having the determined transmitting frequency, and controls the power supply circuit to generate the transmitting power having the determined transmitting frequency and voltage.
 7. The power transmitter apparatus according to claim 6, further comprising a capacitor connected to the power transmitting coil so as to form an LC resonant circuit.
 8. The power transmitter apparatus according to claim 6, further comprising a magnetic core around which the power transmitting coil and the auxiliary coil are wound, wherein the auxiliary coil is disposed so as to surround the power transmitting coil.
 9. A contactless power transmission system including: a power transmitter apparatus; and a power receiver apparatus comprising a power receiving coil, wherein the power transmitter apparatus comprises: a power transmitting coil; an auxiliary coil electromagnetically coupled to the power transmitting coil; a power supply circuit that generates transmitting power having a variable voltage and a variable frequency and supplies the transmitting power to the power transmitting coil; and a control apparatus for the power transmitter apparatus, wherein the power transmitter apparatus transmits power to the power receiver apparatus in a contactless manner, wherein the control apparatus comprises: a detection circuit that detects at least one value of a value of a current flowing through the power transmitting coil, and a value of a current or voltage generated by the auxiliary coil; and a control circuit that determines a transmitting frequency based on the at least one value detected by the detection circuit, the transmitting frequency at least locally minimizing dependence of an output voltage of the power receiver apparatus on a load value of the power receiver apparatus, determines a voltage for the transmitting power at which the output voltage of the power receiver apparatus is equal to a predetermined target voltage when generating the transmitting power having the determined transmitting frequency, and controls the power supply circuit to generate the transmitting power having the determined transmitting frequency and voltage. 